Process of production of anhydrous monocalcium phosphate



Jan. 27, 1942. F. B. CARPENTER. JR" ETAL 2,271,361

PROCESS OF PRODUCTION OF ANHYDROUS MONOCALCIUM PHOSPHATE Filed April 25, 1959 Calcium Mdllophnapimie Mmallydmie aoz I13P0 IE. CEI'IDEDZEI; J

a WK.E705.

Patented Jan. 27, 1942 PROCESS OF PRODUCTION OF ANHYDROUS MONOCALCIUM PHOSPHATE Frank B. Carpenter, Jr., and Wilbur K. Enos,

Charleston, S. 0.,

assignors to Virginia- Carolina Chemical Corporation, Richmond, Va., a corporation of Virginia Application April 25, 1939, Serial N0.269,986

3 Claims.

This invention pertains to an improved process for manufacturing monocalcium phosphate and. also for improving the physical and chemical properties of commercial forms of monocalcium monocalcium phosphate monohydrate, and stabilized and non-hygroscopic. A further object is to produce MCP in anhydrous-form, preferably as discrete crystals, and to control the general composition of these particles in commercial man- 55 ufacture. These and other objects will appear hereinafter more fully in an illustrative description of this invention taken with the accompanying drawing and more fully pointed out in ed with concentrated phosphoric acid at a temperature below about 300 F., and subsequently while heating gradually to 300 to 400 F. agitating and gradually adding lime until the material containsno vfree acid and is powdery and fluphosphate. the appended claims.

Monocalcium phosphate is the primary salt of In the drawing: orthophosphoric acid. It is known in two modifi- Figure 1 illustrates in diagrammatic form apcations, the anhydrous salt Ca(H2PO4)2 which paratus that is capable for use in the process of exists in the triclinic crystal system, and the this invention; monohydrate C3(H2PO4)2H2O which crystallizes in Figure 2 illustrates somewhat conventionally in the rhombic system. The pure products are crystalline particles of the anhydrous MCP which hygroscopic in nature and also are easily hyit is the object of this process toproduce; drolysed into the more stable dicalcium salt and Figure 3 represents a diagram of various free phosphoric acid. The pure crystal products physical-chemical phase conditions involved are cohesive in nature and not free flowing. In in during certain operations of the present process. commercial practice it is common to stabilize According to the present invention, a suitable monocalcium phosphate, herein designated as process to produceanhydrous MCP in a stable MCP, by adding calcium compound in excess of and fluent form comprises mixing an alkali earth that required for the mono-salt, so that some dibase, particularly calcium oxide or hydroxide, and tricalcium phosphate are formed as in U. S. with phosphoric acid in certain amounts, at cer- Patent 1,913,796. It is also common practice to tain temperatures and in controlled manner, as coat the particles with basic tricalcium phosphate will be more fully pointed out hereinafter. Prefas shown in 142-7, (1933) Moss, Schilb & erably, such control is employed as maintains the Warning Ind. Eng. Chem, magnesium commagma during reaction within certain definite pounds, as in U. S. Patents 1,913,796 and 1,785,473 2.5 temperature ranges and certain compositions to harden the crystals, lime, as in U. S. Patent until a fluent powder results. The production is 2,018,449 or 2,135,475 and othersubstances to accomplished in a single mixing operation. eliminate traces of free acid, and to make the In the drawing, which refers to specific forms .resultant products more free flowing. In coatof this invention without limitation, Figure 1 deing the particles these materials retard the rate picts the mixer used for this purpose. A shell of reaction. This principle of retarding the rate I is lined with acidproof brick and is mounted of reaction has also led to the development of for rotation about a vertical axis. Theequipment inert coatings such as casein, oils, and similar is the typical chemical plastic mixer employed in materials. the industry in many modifications. Within the A principal use of commercial MCP is as an shell there are two agitators with mixing blades acid leavening ingredient. In this use the prodrevolving relatively to the shell. A plow 3 net reacts with bicarbonate 'of soda and releases throws the material against the agitators. A carbon dioxide as leavening gas. The amount of cover 4 admits the solid material and liquid rebicarbonate of soda that is neutralized by the agent; and through the cover is a vent 5. The monocalcium phosphate is known as the, neu-40 material is discharged through a central lower tralizing value of the product. The stability of outlet 6 to which the material is thrown 'by the commercial MCP and the rate of reaction are plow. Located conveniently near the mixing pan points of prime importance. is an acid boot with provisions for heating and An object of the present invention is to promeasuring the acid to be added to the shell. This duce in a single operation and simplemanner a boot and all piping therefrom may be rubber lined stable monocalcium phosphate free from water to prevent contamination of acid by corrosion. of crystallization and containing no free acid. In illustrative and preferred practice under the A particular object is to control the particle size. process of this invention, though this invention An additional object is to obtain a monocalcium is not necessarily limited to these illustrative dephosphate with higher neutralizing value than tails, a suitable amount of powdered lime is heatent. In general, the crystals form initially in a medium more acid than pH 3.8, and the medium gradually is brought to a condition less acid than pH 4.3. Preferably, calcium oxide in pure condition where a food product is to be obtained is first introduced into the mixer. In lieu of calcium oxide, calcium hydroxide, dior tricalcium phosphate, or calcium carbonate may be employed. The lime may well be of a fineness of 100 mesh.

For example, 284 pounds of lime are placed in the mixer and this mass brought approximately under the acid inlet. This step is not essential to the process, but has been found to secure an effective distribution of the solid in the liquid during the early stages of the ensuing chemical reaction. Then 1872 pounds of phos phoric acid, about 83% HaPO4, heated to a temperature of 120 F. is added. The reaction is exothermic and proceeds with considerable evolution of heat which is utilized to control the reactions. Initially the reaction is slow until the mass approaches the boiling point of the mixture. It then proceeds rapidly with evolution of water vapor. Some loss of lime may occur with the vapors. The magma is first of a pasty consistency, then it becomes very fluid, and with the evolution of water vapor again becomes pasty. In composition the mass is initially unreacted lime, with calcium phosphate monohydrate crystals and an excess of phosphoric acid; and then as the temperature rises above the critical points, later discussed, the mass becomes a mixture comprising the anhydrous MCP crystals and phosphoric acid. More lime, 142 pounds, are now added gradually while the mixer revolves. This is the critical stage of the formation and growth of the anhydrous MCP crystals. The mix now contains 90-98%, preferably about 96%, of the theoretical amount of Get) for the mono salt of orthophosphoric acid. The temperature is held preferably within the range 300 and 340 F. by control of the rate of addition of the lime, and the speed of the mixer. The temperature in the latter part of this stage may be permitted to attain 375 F., but temperatures of approximately 400 F. and above may result in some formation of calcium acid pyrophosphate or of the polyphosphates. At lower temperatures than in the desired range, there is growth of monohydrate crystals, which are not redissolved. The total amount of the lime utilized varies with the loss by entrainment with the steam. The product in the pan now consists of essentially pure MCP anhydrous crystals with some free phosphoric acid.- Samples .of the product are taken at this point to determine the amount of free phosphoric acid remaining in the product, and also the neutralizing value is tested to indicate the presence either of the monohydrate or of the pyroand other salts. During this stage about 85% of the final amount of lime has been utilized and the 7 weight ratio of CaO:P2O5 is about 0.38.

To eliminate color in the MCP due to traces of organic matter in phosphoric acid, sodium chlorate is added to the acid and to the mixing operation above described. One of the resulting products in hot phosphoric acid is sodium phosphate. We have found that if the NazO content of the phosphoric acid is held within the range of -15%, as for example 0.85%, the anhydrous crystal MCP obtained by the method above can be more easily stabilized. With the use of pure phosphoric acids where there is less need for sodium chlorate, then soda may be added as the hydroxide, the carbonate, or the bicarbonate.

Potassium chlorate, potassium hydrate, or potassium carbonate can also be used in likev lime as above described; with further mixing the product becomes a very fluent crystalline mass. Additional lime of a fineness of all minus 100 mesh and preferably of about minus 150 mesh, is now added. Thus added it reacts with traces of phosphoric acid still remaining, and stabilizes the resultant product. The exact amount to be added varies with the results of the control analysis. In the illustrative mix, 90 pounds have been found to be the proper quantity. This is added very slowly while the mixer is continued at speed of approximately 6 R. P. M., 8 foot diameter mixer used. Temperature control is essential as above set forth, and the temperature is held preferably at 375 F. The purpose is to obtain the maximum coverage in the least time, at a temperature not exceeding about 375 li t; with agitation at the maximum rate available. With the addition of this lime there is an initial rise of temperature, and then the mass begins to cool. When the temperature has fallen to approximately 300 F., or lower, approximately 15 pounds of powdered basic tricalcium phosphate are added to improve the mechanical condition of the crystals. The mass is now dry and exceedingly fluent, and when cooledis easily handled.

The resultant product of the process consists of triclinic anhydrous MCP crystals, stabilized and made free flowing by a surface coating of essentially calcium oxide and calcium phosphate bound to the crystal apparently by the phosphate of the alkali metal. The crystals are translucent. However, under high magnification the basic stabilizers may be seen adhering to the crystal surfaces. The crystals produced are substantially from 20 to microns in length and 5 to 15 microns in width. The mechanical condition is such that the angle of repose of the material is approximately 36. The composition of the material is essentially that of anhydrous MCP crystals modified by the alkaline stabilizers employed. Its neutralizing value is about 88. The material in a strength of 0.5% solution exhibits an initial pH of 4.1 which on standing increases to 4.8. The CaO:P2O5 weight ratio in theoretical MCP is 0.39, ln'the usual commercial MCPs is from 0.40 to 0.44 and in this product may be 0.42 to 0.46.

In general, it is recognized that the text A Comprehensive Treatise on Inorganic and Theoretical Chemistry by J. W. Mellor, volume III, 1923, page 887 describes anhydrous calcium dihydrophosphate and page 865 cites work by H. Bassett, Zeit, Anorg. Chem. vol. 59, 1, 1908. Some description of the system PzO5CaO-H2O is given by N. A. Clark in Jour. Physical Chemistry 35 1232 (1931).

In operating the process of this invention, it is convenient to describe suitable conditions by reference to Figure 3 which is part of a so-called triangular diagram of the system P205 CaO-HzO. Thus, Figure 3 shows that at different temperatures, for example, from 122 F. to 304 F., there are series of compositions represented by lines of demarcation, which may be varied. For example, decrease of P205 yields" calcium monophosphate monohydrate or increase of P205 and increase of CaO yield dicalcium phosphate. In the process of the present invention it is desirable to operate, generally speaking, in the area of composition on that side of the compositional lines lying toward thev P205 apex.

Thus, for example, as startin reagents in the process of this invention it is desirable to select phosphoric acid at a point indicated by the arrepresented in general by progress on the diagram of Figure 3 from the I-IzO-PzOs base toward the CaO apex to areas limited by the appropriate compositional lines at the temperatures indicated. In proceeding with the process of the present invention, the operating temperatures, as have been stated, lie above 300 F. and arenot shown on the diagram of Figure 3 inasmuch as their representation would entail the use of a solid diagram of more complicated nature than is feasible or necessary in the present de-' scription. Figure 3, serves, however, to illustrate initial conditions which should obta at the outset of the practice of this process. owever. in the practice of this process the addition of lime is-restricted so as not to exceed at "any time during the stage of formation of the anhydrous" monocalcium phosphate the composition represented on Figure 3 at the point A, which is approximately 24% CaO. This is the stage during which the magma is liquid or pasty. Subsequently, during the stageof final growth of crystals and during the stage of stabilizing treatment while the material is becoming more dry and powdery the percentage of calcium oxide may exceed 24%.

It will be recognized that the practice of this invention is not limited to any theory nor to 4 illustrative details shown here and that the facts presented in Figure 3 permit some latitude of initial choice of operations.

While according to the provisions of the patent statutes we have described the general principles of this invention and illustrated them with reference to procedures that are at present preferred, it is, evident that now those skilled in the art will be able to select variations in conditions and procedure within the scope of the appended claims.

What we claim is:

1. A process of producing monocalcium phosphate in the form of small discrete crystals, comprising heating phosphoric acid of atleast 75% H3PO4 while adding gradually an amount of lime less than about %of that necessary to convert the H3PO4 to monocalcium phosphate until the mass thickens with formation of hydrated crystals of monocalcium phosphate, and then heating to bring the mass into a more liquid and definitely boiling condition at about 300 F., with conversion of the hydrated mono calcium phosphate to the anhydrous crystalline form, and then eliminating liquid from the mass by heating and adding lime to attain a weight ratio of CaO:P2O5 of about 0.39 to complete the formation of anhydrous monocalcium phosphate, free from excess phosphoric acid, as a dry product.

2. A process of producing monocalcium phosphate in the form of small discrete crystals, comprising heating phosphoric acid of at least ,HaP04,while adding gradually an amount of lime less than about 65% of that necessary to convert the HaPO4 to monocalcium phosphate until the mass thickens with formation of hydrated crystals of monocalcium phosphate, and then heating to bring the mass into a more liquid and definitely boiling condition at about 300 F., with conversion of the hydrated monocalcium phosphate to the anhydrous crystalline form, and then eliminating liquid from the mass by heating and by gradual addition of lime to attain a weight ratio of CaOzPzOa of about 0.39, and then, maintaining a temperature of substantially 375 F., finally eliminating all free phosphoric acid byadding lime to a weight ratio of about 0.42,, to obtain a fluent mass of anhydrous crystals as .a dry product.

3. A process of preparing stabilized monocalcium phosphate free from water of crystallization comprising heating phosphoric acid of at least 77% HaPO4 from a temperature of about F. with calcium baseiin an initial amount less than 65% of the CaO equivalent to monocalcium phosphate, raising the temperature.

gradually with. formation of hydrated monocalcium phosphat until the mixture boils, maintaining a freely boiling liquid condition at substantially 300 F. until the hydrated monocalcium ,phosphate is converted to the anhydrous crystalline form, and then gradually adding lime and agitating the mixture at about 320 F. to 340 F. until the mixture is powdery, then adding finely divided lime until the weight ratio of CaO:PeOs .'is about 0.42 and the product is powdery and fluent.

FRANK B. CARPENTER, JR. WILBUR K. ENOS. 

